The present invention relates generally to a remote controlled security system and in particular to a security system using synchronized one-way transmissions wherein synchronization is maintained between the transmitter and receiver even after inadvertent activations of the transmitter.
Many security systems are available where a pushbutton remote transmitter enables a function in the vehicle such as unlocking the doors or turning off an alarm. Such systems may have the remote control unit emit a fixed identification code exclusive to the transmitter/receiver combination. However, a security system using a fixed code can be defeated by using a random scanning device which transmits random codes in rapid succession in hopes of reproducing a valid code. Security is increased in such a system by increasing the number of combinations in the security code, i.e., increasing its length.
The fixed code system can also be defeated using a recording device which captures and then duplicates transmissions of the security code from a valid transmitter.
Other security systems are known which use a security code that changes after each transmission, thereby eliminating the possibility of a transmission being recorded and retransmitted because an exact duplicate of the previously transmitted signal will be accepted by the receiver.
One known method of changing the security code uses a look-up table in both the transmitter and receiver which is indexed every time a transmission is made. One problem with such a system is that if the transmitter is operated outside the receiver range, the transmitter will no longer be synchronized with the receiver and will be unable to control the system. To alleviate this synchronization problem, two-way transmissions using a "handshaking" technique have been used to ensure that the transmitter and receiver are indexed simultaneously. However, the need to have both a transmitting and a receiving capabilities in the remote control unit and the in-vehicle apparatus is undesirable.
A security system described in U.S. Pat. No. 4,847,614 employs a one-way transmission method. A digital code is stored in the system memory and is changed in both the transmitter and receiver by using digital logic gates to operate on the bits to produce the next code of the sequence after each transmission. This system also suffers from synchronization problems which if severe enough, will result in the operator using a key to manually unlock the door. If the system is less than 10 steps out of sink, the system automatically resynchronizes itself. If the system is between 10 and 256 steps out of sink, two successive transmissions must be made. This situation would happen quite frequently resulting in a perception of poor performance from the customer. Furthermore, after changing the transmitter battery, the transmitter will have lost synchronization.
The security system described in U.S. Pat. No. 5,159,329 uses a security code portion which has a fixed portion and a portion which varies according to a specified algorithm. The system advances the alterable portion of the code word after each transmission. The problem with such a system is that once the transmitter is activated outside the range of the control device the transmitter and control device are then out of synchronization. The system can be reset by the use of a mechanical switch inside the vehicle. This situation would happen quite frequently and would be perceived as poor performance by the customer.
It would be desirable to have a remote control system using one-way transmissions and a rolling code that is not dependent on the synchronization of the transmitter and receiver.